Runtime Debugging

Anyone who has been programming longer than a week knows that
getting the syntax of your programming language right is the
easy part of debugging. The hard part comes after that,
when you need to understand why your syntactically correct program
doesn't behave as you expect.

The Unix tradition encourages developers to anticipate this
problem by designing for transparency — in particular, designing
programs in such a way that their internal data flows are readily
monitored with the naked eye and simple tools, and readily mentally
modeled. This is a topic we covered in detail in
Chapter 6. Design for
transparency is valuable both for preventing bugs and for easing the
runtime-debugging task.

Design for transparency is not, however, sufficient in itself.
When you are debugging a program at runtime, it's extremely useful to
be able to examine the state of your program at runtime, set
breakpoints, and execute pieces of it down to the single-statement
level in a controlled way. Unix has a long tradition of hosting
programs to help you with this. Open-source Unixes feature a powerful
one called gdb (yet another
FSF
project) that supports C and C++ debugging.

Perl,
Python,
Java, and Emacs
Lisp all
support standard packages or programs (included with their base
distributions) that allow you to set breakpoints, control execution,
and do general runtime-debugger things. Tcl, designed as a small language for small
projects, has no such facility (though it does have a trace facility
that can be used to watch variables at runtime).

Remember the Unix philosophy. Spend your time on design quality, not
the low-level details, and automate away everything you can —
including the detail work of runtime debugging.